need to collect balance of past due promissory note.

Sold a full liquor bar for past due bills and a promissory note for $20,000.00 He stopped paying. I sent a (2) registered demands for full remaining balance of just over $17,000.00. I recvd one back. The one sent to him at the business was accepted and the one sent to his home was not accepted. I just recv a small soc sec mo pmt. I need my money. I need glasses and tires for my 14 yr old car. I am a senior citizen with low income. The promissory note says that he is responsible for resanable collection costs.